Most controls that show a drop down capture the mouse to cancel the scrolling of any parent ScrollViewers while the drop down is opened. The ToolBar overflow items drop down doesn't prevent the scrolling in parent ScrollViewers (a ScrollViewer that wraps the content where the ToolBar is defined). In that case if the user scrolls the ScrollViewer while the overflow items are opened, the ToolBar control moves according to the new position, but the popup stays open at its original position.

The common behavior here is to prevent scrolling while the drop down is opened. 

To work this around, you can subscribe to the MouseWheel event of RadToolBar and handle it in case the IsOverflowOpen property is True.

private void ToolBar_MouseWheel(object sender, MouseWheelEventArgs e)
        {
            var toolBar = (RadToolBar)sender;
            if (toolBar.IsOverflowOpen)
            {
                e.Handled = true;
            }
        }

When the RadToolBar is collapsed through the OverFlow Popup, the popup remains open. 

As a workaround the popup can be closed when the RadToolBar's Visibility is collapsed:

The button focus visual should not be displayed when you click on another button in the UI. Currently, because the toolbar has its own focus scope, it is possible to have the focus on the button and also on another UI element. This means that the button will have its focus visual shown.

To work this around you can extract the Template of the RadButtonStyle from the Telerik.Windows.Themes.Fluent.xaml file and remove the General IsFoucused setters (MultiTrigger). Then you can apply the style using the style selector of the toolbar. For your convenience I prepared a small example showing this approach. I hope that helps.

https://docs.telerik.com/devtools/wpf/styling-and-appearance/styling-apperance-editing-control-templates

https://docs.telerik.com/devtools/wpf/controls/radtoolbar/styling-and-appearance/style-selector

The RadToolBar.OverflowMode attached property is not respected when set on an element in the ItemTemplate of RadToolBar.

Once the Windows8Touch theme is selected changing the theme to anyone else makes the Toolbar in the selected theme to look large like in Windows8Touch theme

Setting ToolBar.Background to Transparent should make the control transparent

Allow buttons to be right aligned in the toolbar

Available in Q3 2014.